1. Field of the Invention
The present invention relates to a transmission having a hydrostatic unit and a charge pump and in particular to a transmission having a cold start valve that returns oil from the pump outlet to the pump inlet to avoid pump pressure buildup during cranking of an associated engine until after the engine has been started.
2. Description of Related Art
An infinitely variable hydro-mechanical transmission may be directly coupled to a driving engine without a clutch therebetween to mechanically disconnect the transmission from the engine. Such a transmission typically has an oil pump to provide charge oil to the hydrostatic unit of the transmission. The charge pump produces a torque load on the engine that must be overcome during engine starting to crank the engine. The torque load with cold oil is significantly higher than with warm or hot oil. During cold weather starting, the torque load of the charge pump can exceed the torque available for engine cranking, thus preventing the engine from starting.
The above problem during cold starting is overcome by the provision of the cold start valve in the transmission of the present invention. The cold start valve dumps the charge pump oil to the sump, i.e. to the pump inlet, for a period of time while the engine is cranked. The dumping of the charge oil eliminates or drastically reduces the pump load on the engine thereby reducing the torque required to crank the engine. The cold start valve is a mechanical valve and operates each time the tractor engine is started. The valve is temperature sensitive so that at lower temperatures, the charge pump oil is dumped for a longer period of time than at higher temperatures. There is no operator intervention required, no electronics are required and there is nothing for the operator to manually disconnect or reconnect for cold starting.